EFFECT OF THE DIRECT EXCHANGE INTERACTION BETWEEN MAGNETIC IMPURITIES ON MAGNETIZA TION IN DILUTED MAGNETIC SEMICONDUCTORS

EFFECT OF THE DIRECT EXCHANGE INTERACTIONBETWEEN MAGNETIC IMPURITIES ON MAGNETIZATION IN DILUTED MAGNETIC SEMICONDUCTORS VU KIM THAI, HOANG ANH TUAN Institute of Physics, VAST LE DUC ANH

EFFECT OF THE DIRECT EXCHANGE INTERACTION

BETWEEN MAGNETIC IMPURITIES ON MAGNETIZATION

IN DILUTED MAGNETIC SEMICONDUCTORS

VU KIM THAI, HOANG ANH TUAN Institute of Physics, VAST

LE DUC ANH Hanoi National University of Education

Abstract We consider a model of III-V diluted magnetic semiconductors where both of the ex-change interaction between carrier and impurity spins, and the direct exex-change interaction between magnetic impurities are taken into account The magnetization as a function of temperature for

a wide range of model parameters is calculated and discussed We show that for a degenerate carrier system the suppression of the magnetization is sensitive to the antiferromagnetic coupling constant and the impurity concentration.

I INTRODUCTION The DMS combine ferromagnetism with the conductivity properties of semiconduc-tors Therefore, they are ideal materials for applications in spintronics where not only the electron charge but also the spin of the charge carrier is used for information pro-cessing For instance, they allow to resolve the conductivity mismatch problem which hinders a high polarizability of injected electrons in a ferromagnetic metal/semiconductor junction [1]

One prominent DMS is Ga1−xMnxAs (typical x ≈ 1 − 10%) with the Mn ions substitutionally replacing Ga at the cation sites Mn ions in Ga1−xMnxAs serve a dual purpose, acting both as acceptors and as magnetic impurities, whose spins align at the ferromagnetic transition [2] Since it is widely believed that the carriers are mediating fer-romagnetic interaction, ferromagnetism in DMS is called carrier-induced ferromagnetism and several theories addressing the mechanism are already available [3] Some mean-field theories, based on the RKKY interaction [4-5] have succeeded in explaining some charac-teristic and transport properties of DMS’s However, further results show that for a low doping level the RKKY interaction between localized spins is insufficient [6-7] In this case the impurity band model, where p holes move around interacting with localized spins

at Mn sites through the antiferromagnetic exchange interaction, is a widely used model for (III,Mn)V-type DMS [8-9] In almost theoretical works above, however, the effects of super-exchange interaction between d-electrons in Mn have been neglected

From the application point of view, the low Curie temperature of the investigated DMS represent a serious problem, and many efforts have been devoted to find DMS with higher Tc On the other hand, the temperature dependence m(T ) of the spontaneous magnetization possesses many important characteristics, such as concavity/convexity of

the curve, value of saturation magnetization, etc., so its study has a very high potential for elucidating the physics behind DMS ferromagnetism in real systems The purpose

of this paper is to calculate the magnetization as a function of temperature for a wide range of model parameters including the antiferromagnetic coupling constant between magnetic impurities We show that for a degenerate carrier system the suppression of the magnetization is sensitive to the antiferromagnetic coupling constant and the impurity concentration

II THE MODEL AND FORMALISM

We consider the following model of III-V DMS A1−xMnxB where both of the ex-change interaction between carrier and impurity spins, and the direct exex-change interaction between magnetic impurities are taken into account

ijσ

tija+iσajσ+X

i

ui− J X

<ij>

~

where ui is either uA

i or uM

i depending on the ion species occupying the i site:

ui =







EAP

σ

EMP σ

a+iσaiσ− ∆P

σ

a+iσaiσ(σSi), i ∈ Mn (2)

Here a+iσ(aiσ) is the creation (annihilation) operator for a carrier with spin σ at i site;

~

Si denotes the spin of localized impurity at i site ; ∆ is the effective coupling constant between the localized spin and itinerant spin; J is the coupling constant between the neighboring localized impurity spins, which depends on their distance and for the AF exchange interaction case J < 0 To consider the effect of the direct exchange interaction between magnetic impurities on magnetization, we simply the problem, dividing equation (1) into the impurity term and the itinerant carrier term

Himp = −X

i

hSiz− J X

<ij>

Hcarr =X

ijσ

tija+iσajσ+X

i

where h is the field induced by the polarization of the carrier spins In this study we treat the localized spin as the Ising spin (Siz = ±1) and treat the Himp in the molecular field approximation as Sz

iSz

j =< Sz

i > Sz

j+ < Sz

j > Sz

i− < Sz

i >< Sz

j > Within this mean approximation, the Hamiltonian (3) becomes

HimpM F = N xJγm2−X

i

where N is the number of lattice sites, x is Mn density, m =< Sz

i > refers to the average magnetization per lattice site, γ is the number of the neighboring localized impurity spins

located around a given one With simplified Hamiltonian (5), we obtain the partition function

Zimp= (e−βJ γm2 X

S z

=±1

eβ(h+2Jγm)Sz

where β = 1/kBT The free energy for the localized spin system is then given as

Fimp= −kBT lnZimp= N xJγm2− NxkBT ln( X

S z =±1

eβ(h+2Jγm)Sz) (7)

We apply CPA [8,10] to the Hamiltonian (4) In CPA the carriers are described as inde-pendent particles moving in an effective medium of spin-deinde-pendent coherent potentials The coherent potential Σσ (σ =↑, ↓) is determined by demanding the scattering matrix for a carrier at an arbitrarily chosen site embedded in the effective medium vanished on average By using a bare semicircular noninteracting density of states (DOS) with half-bandwidth W : ρ0(z) = πW22√

W2− z2 we obtain the following equation for the Green function for a given magnetization m

Gσ(ω) = 1 − x

ω − wGσ(ω) − EA

ω − wGσ(ω) − EM + ∆σ+

x(1 − m)/2

ω − wGσ(ω) − EM − ∆σ, (8) where w = W2/4 and σ = ±1

The Eq (8) is easily transformed into a quartic equation for Gσ(ω) and it is solved analytically by using Farrari method Throughout this work, we assume that the carriers are degenerate Then the carrier density and energy can be expressed as

n =

µ

Z

−∞

Ecarr(m) =

µ

Z

−∞

where µ is the chemical potential and ρσ(ω) = −π1=Gσ(ω) is the DOS with spin σ The free energy per site of the system (1) at temperature T is given as

F (m) = Ecarr(m) + hmx + xJγm2− xkBT ln( X

S z =±1

eβ(h+2Jγm)Sz) (11)

By minimizing F with respect to m we obtain the following equation for h

h = −1 x

dEcarr(m)

By using the Weiss molecular field theory, each impurity spin feels an effective field h + 2Jγm and thus we have

Equations (8)-(10), (12) and (13) form a set of self-consistent equations for µ and m for a given set of parameter values x, n, ∆, J, EA, EM and T

Fig 1 Temperature dependent magnetization for various antiferromagnetic

cou-plings for x = 0.05, n = 0.025, EM = −0.2, ∆ = −0.3.

III NUMERICAL RESULTS AND DISCUSSION Through this work we take EAas the origin (= 0) and W as the unit of energy, γ = 6 for simple cubic lattice Before numerical solving the equations (12) -(13), let us briefly consider limiting case In the absence of the direct exchange interaction between magnetic impurities, setting J = 0, ∆ = −0.4 and EM = −0.3 in (13) we reproduce the CPA result for the magnetization of Ga1−xMnxAs obtained by Takahashi et al [11] In addition, since tanhx is an increasing function from Eq (13) it is easily seen that taking into account the antiferromagnetic interaction between localized spins (J < 0) leads to decreasing the magnetization m(T ) We turn now to present our numerical results In Fig 1, we show our calculated magnetization of the local moments as a function of temperature for different values of J = 0, −4, −8 and −12.10−4, for x = 0.05, n = 0.025, EM = −0.2 and

∆ = −0.3 One can see that the ferromagnetism is always preferable at low temperatures and for fixed x, n, EM, ∆ and T the magnetization decreases with increasing |J| This constant depends on the distance between two neighbour impurities, so it depends on the impurity concentration x Unfortunately, as noted in [12], non of J neither x, n, EM of our model is directly experimentally measurable That is why a detailed comparison between our result and experiment cannot be done Here we choose the magnitude of J in the same order as in Ref.[12] These results indicate that the neighboring magnetic impurities not only couple anti-ferromagnetically to each other but also reduce the carrier-induced ferromagnetic interaction In Fig 2, the temperature dependence of magnetization is plotted for x = 0.05, n = 0.025, EM = −0.2, J = −4.10−4 and for several values of

∆ = −0.3, −0.4 and −0.6 Here, the fact that our m(T ) and Tc increases with increasing

|∆| for all ∆ is due to the Weiss mean field theory Fig 3 displays the change of the magnetization with the change of nonmagnetic potential Comparing with the curves

in Fig 2 it is clear that EM simply renormalizes the effective value of ∆ Fig 4 and

Fig 2 Temperature dependent magnetization for various effective coupling

con-stants for x = 0.05, n = 0.025, EM = −0.2, J = −4.10 −4

Fig 3 Magnetization for different values of nonmagnetic potential for x =

0.05, n = 0.025, ∆ = −0.3, J = −4.10 −4

Fig 5 show the magnetization m(T ) for various value of x and n We find that the magnetization is sensitive to the impurity concentration x and m(T ) is maximized for the carrier density roughly half of the concentration of localized spins (n/x ≈ 0.5) which agrees with [9,12] However, unlike Ref [12] where the magnetization curve is concave for low carrier densities, our curve is convex for all values of n

To summarize, applying the CPA and mean-field approximation we have studied the effects of the direct exchange interaction between magnetic impurities on magnetization

in DMS (III,Mn)V-type The magnetization as a function of temperature for a wide range

of model parameters is calculated and discussed We have shown that for a degenerate carrier system the suppression of the magnetization is sensitive to the antiferromagnetic coupling constant and the impurity concentration This result also implies that the super-exchange interaction between d-electrons in Mn has a tendency to reduce Tc which will be investigated in the near future

Fig 4 Temperature dependent magnetization for various magnetic impurity

con-centrations for n = 0.02, EM = −0.3, ∆ = −0.3, J = −4.10 −4

Fig 5 Magnetization for various values of carrier density for x = 0.05, EM =

−0.2, ∆ = −0.3, J = −4.10 −4

ACKNOWLEDGMENT This work is supported by the National Foundation for Science and Technology Development (NAFOSTED)

REFERENCES

[1] G Schmidt et al., Phys Rev B 62 (2000) 4790 (R).

[2] H Ohno, J Mag Mag Mat 200 (1999) 110.

[3] T Jungwirth et al., Rev Mod Phys 78 (2006) 809.

[4] G Zarand, B Janko, Phys Rev Lett 89 (2002) 047201.

[5] T Dietl, H Ohno, F Matsukura, Phys Rev B 63 (2001) 195205.

[6] G A Fiete at al., Phys Rev B 71 (2005) 115202.

[7] R Bouzerar et al., Phys Rev B 73 (2006) 024411.

[8] M Yagi, Y Kayanuma, J Phys Soc Jpn 71 (2002) 2010.

[9] M Takahashi, K Kubo, J Phys Soc Jpn 72 (2003) 2866.

[10] Hoang Anh Tuan, Le Duc Anh, Commun Phys 15 (2005) 199.

[11] M Takahashi, N Furukawa, K Kubo, J Mag Mag Mat 272-276 (2004) 2021.

[12] S Das Sarma, E.H Hwang, S A Kaminski, Phys Rev B 67 (2003) 155201.

Received 30-09-2011